During development of the central nervous system ("CNS"), multipotent precursor cells, also known as neural stem cells, proliferate, giving rise to transiently dividing progenitor cells that eventually differentiate into the cell types that compose the adult brain. Neural stem cells are classically defined as having the ability to self-renew (i.e., form more stem cells), to proliferate, and to differentiate into multiple different phenotypic lineages, including neurons, astrocytes and oligodendrocytes.
These neural stem cells have been isolated from several mammalian species, including mice, rats, pigs and humans. See, e.g., WO 93/01275, WO 94/09119, WO 94/10292, WO 94/16718 and Cattaneo et al., Mol. Brain Res., 42, pp. 161-66 (1996), all herein incorporated by reference.
Human CNS neural stem cells, like their rodent homologues, when maintained in a mitogen-containing (typically epidermal growth factor or epidermal growth factor plus basic fibroblast growth factor), serum-free culture medium, grow in suspension culture to form aggregates of cells known as "neurospheres". In the prior art, human neural stem cells have doubling rates of about 30 days. See, e.g., Cattaneo et al., Mol. Brain Res., 42, pp. 161-66 (1996). Upon removal of the mitogen(s) and provision of a substrate, the stem cells differentiate into neurons, astrocytes and oligodendrocytes. In the prior art, the majority of cells in the differentiated cell population have been identified as astrocytes, with very few neurons (&lt;10%) being observed.
There remains a need to increase the rate of proliferation of neural stem cell cultures. There also remains a need to increase the number of neurons in the differentiated cell population. There further remains a need to improve the viability of neural stem cell grafts upon implantation into a host.